From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Summary of a preliminary test on a modified piston crown for the Wraith and Bentley V engines.
| Identifier | ExFiles\Box 148\1\ scan0031 | |
| Date | 17th December 1938 | |
| 1256. To By.{R.W. Bailey - Chief Engineer} from Rm{William Robotham - Chief Engineer}/CTC. c. to Hs.{Lord Ernest Hives - Chair} c. to La.{L. A. Archer} c. to La{L. A. Archer}/Jnr.{Charles L. Jenner} c. to Da{Bernard Day - Chassis Design}/MoS. Rm{William Robotham - Chief Engineer}/CTC.1/JH.17.12.38. PISTONS. The following is a summary of a preliminary test carried out with a modified shape of piston crown. The purpose of the modified shape was primarily to enable one type of cylinder head to be common to both Wraith and Bentley V, the raised portion on the piston in question giving the higher compression ratio for the Bentley engine. It appears from our first test however, that the particular shape we have chosen has another important advantage. Briefly, it allows a higher ratio to be used than that associated with the flat top piston, for the same degree of detonation. Graph No.S.62 shows the power gain between the first and second stages of raising the crown. Graph No.S.63 shows that while we have increased the compression ratio, we have not increased the tendancy to detonate. In other words, low speed M.E.P. is increased and no increased power loss is evident when avoiding detonation. We are at present waiting for a further supply of pistons, which go a stage further, raising the compression ratio to 8 : 1. A provisional patent has been applied for in regard to this piston, and the following explanation to its value has been put forward. The shape of the raised portion of the crown conforms to the shape of the combustion chamber of the cylinder head in such a way as to enter the combustion space, as the piston reaches the top of its stroke. We believe the effect of this is to delay the burning of the gas which exists over the | ||